1. Field of the Invention
The present invention relates to a method of manufacturing a corrugated metallic pipe and which includes providing a mandrel having an axially extending corrugation having a predetermined shape and height, and inserting the mandrel into a smooth cylindrical metallic pipe and radially expanding the mandrel against cheek means, which surrounds the pipe and has a corrugation at least approximately corresponding to the corrugation of the mandrel, to provide a corrugation on the metallic pipe. The present invention also relates to a corrugated pipe formed by method.
2. Description of the Prior Art
The prior art discloses a plurality of methods and tools for manufacturing corrugated metallic pipes. Such methods and tools are disclosed, e.g., in German patents Nos. 3,035,234; 2,851,944; 2,909,142; 3,224,308; German publications DE-AS 2,407,226; DE-05 2,027,638; DE-OS 3,004,838; European Patent No. 298,852, French Patent No. 2,176,707, British patent publication No. 2,268,429; and U.S. Pat. No. 1,890,039.
According to German patent No. 3,035,234, a shaft with a plurality of corrugation discs supported thereon is inserted into a to-be-corrugated pipe, with the pipe being acted upon from outside by another plurality of corrugation discs, which correspond to the corrugation discs provided inside the pipe but are offset relative to the inside corrugation discs. The outer corrugation discs are likewise mounted on a shaft. The rotatable discs are displaced simultaneously toward each other and are axially displaced on their respective shafts. This method is inapplicable to forming corrugations on pipes with a small cross-section because the corrugation discs carrying shafts should be very strong and, thus, should have increased dimensions so that they would not be deformed by an operational pressure generated during the formation of the pipe corrugation.
According to German Publication DE-AS 2,407,226, a smooth cylindrical pipe is inserted into a matrix having a corrugated wall. Then, an elastic pressing member is inserted into the pipe and is compressed from opposite sides with dies, with the pipe being sectionally deformed. This method is very expensive and is applicable to forming corrugations only in pipes having thin walls.
The German patent publication DE-OS 2,027,638, which corresponds to British patent No. 1,341,774, discloses inserting into a to-be-corrugated pipe of a radially expandable mandrel, with sector-shaped discs acting on the pipe from outside, with the discs being displaced radially against the pipe. The desired corrugation is obtained by simultaneous actions of the inner mandrel and the outer discs.
According to German patents Nos. 2,851,944 and 2,909,142, the final corrugation is formed by forming one corrugation after another by applying inner pressure. This method and a tool for effecting the method are very expensive. However, the advantage of this method in comparison with the previously described methods consists in that the wall thickness of the pipe remains substantially unchanged in the corrugation region.
German patent No. 3,224,308 discloses a tool with which a corrugation hollow is formed on a cylindrical pipe step by step. Shaped members are placed into the corrugation hollows of the preformed pipe from outside. Then, a high hydraulic pressure is generated inside of the pipe subjecting the pipe to an axial pressure force which shortens the pipe, whereby the pipe sections, which lie between the hollows supported by the shaped members are displaced outwardly. This method is likewise very expensive.
According to European Patent No. 298,832, a radially expandable corrugated mandrel is inserted into a to-be-corrugated pipe. Sector-shaped cheek plates having a corrugation corresponding to the mandrel corrugation act on the pipe hammering it. The so obtained corrugation has a wall thickness which, because of stretching of the pipe in the corrugation region, is thinner than the thickness of the non-deformed wall.
French Patent No. 2,176,707 likewise discloses a radially expandable corrugated mandrel. As outer jaws, a plurality of gears, which surround the pipe, are used. The gears are so arranged that they coincide with the mandrel corrugation. The gears are supported on a slide displaceable along the pipe. With this arrangement, thinning of the pipe material in the corrugation region is also unavoidable.
British publication No. 2,268,429 discloses a radially expandable mandrel divided in an axial direction into a plurality of discs the outer surfaces of which form a corrugation. From the outside, the pipe is surrounded with sector-shaped disc-like jaws which, together with the mandrel, form the pipe corrugation. A similar method is disclosed in U.S. Pat. No. 1,890,039.
German patent publication No. 3,004,838 discloses a method for producing a corrugated pipe with a distinctive transverse corrugation. The device for forming the corrugation includes a plurality of axially spaced jaws. A to-be-corrugated pipe is inserted into the device, with the outer surface of the pipe engaging the jaws. After the pipe is inserted, it is sealed at its opposite ends, and a high hydrostatic pressure is generated in the interior of the pipe, causing bulging of the pipe sections located between the spaced jaws. Then, press pistons are applied while the high hydrostatic pressure is maintained inside the pipe. This causes folding of the bulged pipe sections. During application of the high hydrostatic pressure in the interior of the pipe, the axial sections of the pipe, which engage the jaws, are stretched out. This causes thinning of the wall in the region of these axial sections resulting in their weakening. During the subsequent axial displacement of the jaws, while the high hydrostatic pressure is maintained, which is necessary not only for further displacement outward of the pipe bulging sections but also for providing an adequate fiction force between the jaws and the pipe sections which engage them, the wall thickness of the jaw engaging pipe sections is further reduced. Thus, the finished corrugated pipe has a smallest wall thickness in the regions of the corrugations hollows. The above-mentioned friction force should be sufficiently high in order to prevent sliding of the jaws relative to the pipe during displacement of the jaws against the small bulge sections of the pipes, which are located between the jaws, which sliding would not permit to obtained a corrugation with predetermined dimensions. Generally, such corrugated pipes are used with steering columns of motor vehicle and serve as intermediate energy-absorbing elements during an accident. The corrugated pipes, which are located inside of the steering columns, serve for transmitting a rotational torque in steering motor vehicles. The corrugated pipe, which is produced with the above-discussed method have the smallest wall thickness in its interior region, and it is this region which is subjected to the highest stress during the transmission of the rotational torque. Therefore, for forming corrugated pipes used in steering columns, pipes with an increased wall thickness are selected so that the corrugated pipe is not subjected to inadmissible stress even in its inner region which is subjected to the highest stresses.
Accordingly, an object of the present invention is a method of manufacturing corrugated pipes with a very distinctive corrugation in which the wall thickness of the corrugated pipe in its interior region is not only retained but is rather increased.
Another object of the invention is a method of manufacturing of a corrugated pipe in which the ratio of a height of a single corrugation protrusion to a length of the corrugation protrusion is approximately 1:1.